Display systems using spatial light modulators use a light source to provide a light that is used to project images onto a display plane. The light source may comprise a single, wide-band light source used in conjunction with color filters to produce light of desired colors. For example, in a three-color display system, color filters may be used to produce light in red, green, and blue wavelengths. In display systems that use other color combinations, different color filters can be used to provide the desired colored light. Alternatively, the light source may comprise multiple, narrow-band light sources that are selected to produce light of desired colors. For example, a three-color display system can include three light sources, with a first light source producing red wavelength light, a second light source producing blue wavelength light, and a third light source producing green wavelength light. Similarly, display systems using a different number of colors will have a different number of color filters or colored light sources.
The chromatic characteristics of the display system, such as the white point, color temperature, and so forth, are determined by the characteristics of the light source in a single light source display system or the individual light sources in a multiple light source display system as well as the color filter in display systems that use the color filter. For example, in a display system that makes use of a color filter, the color filter can be specified by the wavelength characteristics of the individual color filters as well as each individual color filter's transmissivity. Once implemented in the display system, the chromatic characteristics of the display system cannot normally be drastically changed. However, electronic techniques can be used alter the chromatic characteristics of the display system in small degrees.
In addition to correct chromatic characteristics, another factor in the perceived quality of the display system is the display system's bit-depth. The bit-depth of a display system is a logarithmic value of a ratio of a brightest displayable gray shade to a dimmest displayable gray shade of the display system, with a larger bit-depth typically indicating an ability to display a better appearing image. One technique used to increase the bit-depth of a single light source display system is to include a filter that attenuates the brightness of the light projected by the display system onto the display plane. An example of this filter is a neutral density filter. The neutral density filter will typically be used only on the least significant bits (the smallest amounts of light) for a given color, reducing the smallest amounts of light by an additional amount, and thereby decreasing the brightness of the dimmest displayable gray shade and hence, increasing the ratio of brightest displayable gray scale to dimmest displayable gray scale. Another technique to increase a display system's bit-depth is to shorten a shortest displayable amount of light. By reducing the time that a light is projected onto a display plane, the effective amount of light displayed is reduced, thereby reducing the brightness of the dimmest gray scale.
With reference now to FIG. 1a, there is shown a diagram illustrating a color filter 105 for use in a three-color display system with a light source that produces a wide-band light. The color filter 105 is in the shape of a disc with multiple filters, such as a blue filter 110, a green filter 111, and a red filter 112, arranged around its perimeter. As shown in FIG. 1a, the color filter 105 has two filters per color with the filters arranged in a repeated red, green, and blue sequence. As the color filter 105 rotates, one of the filters will always be in front of the light source, filtering the light produced by the light source and producing a light with the chromatic characteristics of the filter. The size of the individual filters, as well as other chromatic characteristics, can be adjusted in order to produce a white light with a desired color point when illuminated with a particular light source. As shown in FIG. 1a, there are two filters per color and the two filters of a given color may be identical to each other or they can be different.
With reference now to FIG. 1b, there is shown a diagram illustrating a color filter 125 for use in a three-color display system with a light source that produces a wide-band light. The color filter 125, like the color filter 105 (FIG. 1a), has two filters per color, such as a blue filter 110, a green filter 111, and a red filter 112. In addition to the six filters, the color filter 125 also includes a neutral density filter 130 for the color green. Since the brightness of the color green has the greatest effect on the overall brightness of the display system, a neutral density filter may be used only for the color green. The neutral density filter can be used to decrease the brightness of the color green and therefore decrease the brightness of the dimmest displayable grayscale. Generally, the addition of neutral density filters for the colors red and blue would only result in an overall reduction in the size of the other color filters in the color wheel, while yielding marginal results. The size of the individual filters in the color wheel 125 are not drawn to scale.
With reference now to FIG. 1c, there is shown a time-space diagram illustrating light output and color filter state for a single frame of a three-color display system using a color filter, such as the color filter 105 (FIG. 1a). The time-space diagram shown in FIG. 1c illustrates light output and color filter state for a single frame duration (span 160). Depending upon the display system, a single frame can comprise two sub-frames (span 165 and span 166) wherein two slightly different images are displayed within a single frame to increase the effective resolution of the display system. The color filter 105 is rotated in front of a light source to provide the desired colors of light, with the order of the sequence of colors being dictated by the order of color filters in the color filter. For example, the rotation of the color filter 105 in a counter-clockwise direction can produce a sequence of blue 170, red 171, and green 172 colors. The speed of the rotation of the color filter can determine the number of times the color sequence appears within a frame (or sub-frame) duration. As shown in FIG. 1c, the blue 170, red 171, and green 172 color sequence appears within a sub-frame duration three times, so the speed of the color filter 105 is referred to as 3 times or 3×. Also shown in the FIG. 1c is the light output of the light source of the display system. Since the display system does not modulate the light output of the light source, the light output of the light source remains at 100 percent (line 175) during the frame (or sub-frame) duration. Although the diagram displayed in FIG. 1c illustrates a display system that utilizes two sub-frames within a single frame, the operation of the color filter 105 would substantially be the same in a display system that does not utilize sub-frames.
One disadvantage of the prior art is that the use of electrical techniques to adjust the color point of the display system can only make small changes to the color point of the display system. Furthermore, the use of the electrical techniques will tend to decrease the overall brightness of the display system, which can negatively impact the image quality of the display system.
Another disadvantage of the prior art is that the presence of the neutral density filter on the color wheel (such as the color wheel 125 (FIG. 1b)) can reduce the overall size of the other color filters on the color wheel. This will reduce the overall ability of the display system to produce the other necessary colors of the display system and thereby negatively affect the image quality of the display system. For example, the presence of the neutral density filter can reduce the overall brightness of the display system and affect the quality of the images being displayed. Furthermore, a single neutral density filter will only be able to provide a single fixed attenuation of the light. To add additional levels of light attenuation will require additional neutral density filters, which will future reduce the size of the other color filters on the color wheel.